1. Field of the Invention
The present invention relates generally to electrical power supplies. More particularly, the present invention relates to cost effective discrete circuit arrangements for providing a trickle voltage and current when a main power supply is unavailable, or for providing auxiliary power.
2. Art Background
Power supplies are undoubtedly the most common operational subsystem encountered in electrical and electronic systems. Power supplies are used to provide virtually all voltages and currents necessary to operate the vast variety of subsystems and peripherals which may exist within any particular electrical or electronic system arrangement. Power supplies may range from direct current (DC) voltage and current supplies to alternating current (AC) and high frequency (HF) supplies, and may comprise systems intended for either high or low outputs.
A power supply used to convert line voltage and current into system voltages and currents for use throughout an electrical or electronic system commonly is referred to as a main power supply. Construction of main power supplies is well known in the art, having been described in the literature for decades. In addition to having a main power supply, an electrical or electronic system may include one or more supplemental or auxiliary power supplies. Such supplemental or auxiliary power supplies produce voltages and currents which regulate or supplement the main power supply. Alternatively, the supplemental or auxiliary power supply may provide voltages and currents when the main power supply is unavailable, for example when the main supply is turned off.
A particular type of auxiliary power supply is a so-called "trickle" power supply which provides low amperage currents and voltages to operate certain devices while the main power supply is turned off. A representative example of a trickle power supply application is the "soft start" feature characteristic of most Macintosh.RTM. personal computers manufactured and distributed by Apple Computer, Inc., Cupertino, Calif. 95014 (Macintosh is a registered trademark of Apple Computer, Inc.). As embodied in the above-mentioned Macintosh personal computers, the soft start feature consists of the main power supply being coupled through an electrically active switching device to the AC mains when a computer user presses a start button located on a computer keyboard. The soft start feature is distinctive in that the main power supply is not coupled to the mains via mechanical contacts in a latching, 2-pole mechanical switch. Rather than feeling a heavy mechanical click as the 2-pole switch on a computer enclosure is engaged, the Macintosh user turning on the computer instead experiences gently pressing the nonlatching start button on the keyboard and the computer thereafter beginning to operate. In the soft start case, the start switch must have power supplied to it even though the main power supply and the computer are turned off.
A prior art trickle power supply arrangement 1 realizing Apple's soft start feature is illustrated in FIG. 1, wherein an unregulated AC voltage is coupled to a laminated line frequency transformer 4 having a primary side and a secondary side. As shown in FIG. 1, transformer 4 converts the higher unregulated voltage (typically 85 to 270 VAC) and current into an appropriate lower voltage and current. The stepped-down lower voltage is typically subsequently accumulated in a capacitor 3 and regulated by an appropriately chosen Zener diode 5 to produce an output trickle voltage and current coupled to a start switch (not shown). In the arrangement 1 illustrated In FIG. 1, the output trickle voltage should be sufficient to drive the start switch, typically comprising approximately 5 volts (V) at an output current of approximately 10 milliamperes (mA). It is possible for transformer 4 to have more than one secondary, each secondary with its own combination of capacitor and Zener as necessary for the design application. For example, a first combination of capacitor and Zener would be dedicated for the trickle supply, and a second different combination dedicated for operating a pulse-width modulator circuit. Alternatively, a self-oscillating flyback or other switching converter arrangement (not shown) may be used in place of laminated transformer 4 in FIG. 1 to reduce the input voltage prior to application to the Zener diode 5. Where a switching converter is used, a one or more transistors will be necessary to control the flyback operation.
In the prior art trickle power supply arrangements discussed above, the cost of the line frequency transformer can account for a substantial portion of the total cost of the trickle power supply. Especially when designing high volume products, e.g., low-cost consumer electrical or electronic items manufactured in the tens or hundreds of thousands of units per month, it is always desirable to achieve a similar or substantially identical result with a lower cost product embodiment. For example, in the above example of personal computers, it would be greatly desirable to be able to produce the trickle voltage and current without requiring the relatively high cost laminated line transformer. Moreover, the laminated line transformer occupies a considerable amount of physical area and volume within the computer relative to other circuit components. As form factors of electronic devices including personal computers continue to shrink, it is invariably necessary to reduce the size of internal components or subsystems within those systems.
As will be described in the following detailed description, the present invention overcomes many of the cost and size problems associated with prior art trickle power supplies by replacing the costly line transformer or switching converter component with an inexpensive but robust SIDAC trigger device. The SIDAC is a known, bidirectionally conducting semiconductor device, and is principally intended for use in arc or gas plasma lamp illumination applications. For example, SIDACs are typically used for generating the high voltage spark necessary to initiate or "strike" the conductive gas plasma reaction in high pressure gas discharge lamps. However, the unique conduction characteristics of SIDAC devices may also be advantageously adapted to the present invention, as will be described in more detail in the following paragraphs. As a result of the replacement of the line transformer with the SIDAC, output trickle voltage and current are produced at substantial cost and space savings. Moreover, the SIDAC-based trickle supply has particular advantages in reducing line frequency radiation in radiation sensitive applications, as is discussed in the paragraphs below.